another problem for new guy in bash

fanbin · 07-27-2007, 11:22 PM

I thought this code "read keylog" could read any thing from my keyboard including the enter. If I type the enter key, what will be stored in keylog? I used to think it will be null, but when I use
[ -n keylog ] to judge if it is null, some mistakes happpen, why?

wjevans_7d1@yahoo.co · 07-28-2007, 12:03 AM

It's easier to answer your question if you give us something specific to hang our hats on.

Please do this:

Write a tiny, complete bash script which illustrates the problem.
Tell us what you expect it to do.
Tell us what happened instead.

When you post the script for us to see, please put it within CODE markers. The easiest way to do that is as follows:

Click the Go Advanced button at the bottom of your editing window.
In the new editing window, highlight the entire script with your mouse.
Click the # icon at the top of this new editing window.

macemoneta · 07-28-2007, 12:10 AM

The 'man bash' documentation will go a long way to explaining things.

Briefly, the 'read' command uses (by default) a newline (the enter key) as a delimiter for the input, so it's not stored in the variable. The test you are looking for is

[ -n "$keylog" ]

Without variable substitution, you were checking the length of the string "keylog" which is not zero.

In addition to the bash man page, there's also the Advanced Bash-Scripting Guide and the BASH Frequently Asked Questions as good sources of information and examples.

fanbin · 07-29-2007, 12:26 AM

Thanks for replying, this is my code.

Code:

read keylog
if [ -z $keylog ]; then
data=1
else 
data=0
fi

What I really want is to ask the programme to check whether I press the enter key, if so, data=1, otherwise data=0, is my code right?

macemoneta · 07-29-2007, 12:35 AM

Yes, but your variable usage is not intuitive. That is, if data=1 there is no data (just enter) and if data=0 there is data. Very confusing to someone else that would have to read your code (that'll be you in a few weeks

).

fanbin · 07-29-2007, 12:54 AM

Thanks, but I still don't get you ? no data?

by the way, the link in your qmd is broken.

Thanks again.

macemoneta · 07-29-2007, 01:05 AM

When writing software, variable=1 is generally used to denote a "true" condition and variable=0 denote a false condition.

In your code when the length of the string is zero (there is no data), you set variable data=1. That is the opposite of the state most programmers would interpret.

OK, I give up. What's a qmd?

fanbin · 07-29-2007, 01:12 AM

ah, I see, that really helps.

qmd , er I mean:

Quote:

Please read: How To Ask Questions The Smart Way. It will save you time and frustration.

qmd is a common phrase in china refering to the things above.

macemoneta · 07-29-2007, 09:00 AM

The link works; it may be blocked by your ISP.

fanbin · 07-29-2007, 09:08 AM

Really thanks. Have a nice day.